Neuromuscular monitoring, the monitoring of muscle relaxation, is as essential as controlling blood pressure or heart rate during surgery. A patient who is extubated when still partially relaxed is at great risk of respiratory complications. Also, a patient incompletely relaxed during surgery can endanger the success of surgery.
Since muscle relaxants are an important cost factor in anesthetic drug selection, neuromuscular monitoring also helps to titrate the exact dosing of muscle relaxants required during surgery. Further applications of neuromuscular monitoring are in intensive care units where peripheral neuropathies with impaired muscle function play an essential role in morbidity of long-term ventilation; repetitive and objective neuromuscular monitoring could help to control and monitor this problem.
Unfortunately, despite these facts, knowledge about the action of muscle relaxants is still quite limited and the tools to measure their function in daily routine are even more limited.
Ideally, (a) neuromuscular function should be easily monitored for all physiologically important muscles in a non-invasive and reliable way, (b) a neuromuscular method and easy-to-use monitoring device should be available to give precise and reliable information about the state of neuromuscular transmission at any given time during surgery, and finally (c) reliable data should be established for any given muscle relaxant on onset, offset and peak effect for different muscles.
During the last 15 years, neuromuscular research and especially neuromuscular monitoring has been the object of important developments. The most important discovery in neuromuscular research has been the understanding that onset, peak effect and offset of neuromuscular blockade after injection of a muscle relaxant are different for different muscles [1]. This is not only due to different circulation times, but also to specific morphological differences of different muscles [2], such as acetylcholine receptor densities and distribution and the type of muscle fibers predominant in a given muscle. This discovery meant that the monitoring of only one, easily accessible muscle, such as the adductor pollicis muscle, was no longer valid to reflect muscle relaxant action in the human body. p The last 15 years were spent to develop methods for monitoring different muscles, such as the larynx, the diaphragm or the eye muscles. This lead to the discovery that the effect of a bolus dose of muscle relaxant at the larynx and diaphragm produces a less pronounced effect in comparison to the adductor pollicis muscle, and a shorter onset and offset of neuromuscular blockade [1]. Although these discoveries were important for research, clinical monitoring of more central, but nevertheless important muscles during surgery (e.g. abdominal surgery) and/or anesthetic relaxation (e.g. intubation) was impaired by the fact that most methods were unsuited for clinical use.
All methods for monitoring muscle relaxation are based on the principle of electric stimulation of a motor nerve and monitoring the reaction of the evoked muscle contraction either directly by measuring the actual force created (mechanomyography) or indirectly by measuring electric potentials at the muscle occurring before the actual muscle contraction (electromyography) or the acceleration of the muscle contraction (acceleromyography).
A fundamental problem of all these research efforts of comparing neuromuscular blockade at different muscles remained that the gold standard of neuromuscular monitoring, mechanomyography which measures the actual force of muscle contraction, cannot be applied to all muscles.
Neuromuscular monitoring using electromyography (measuring the electric potential created by muscle contraction) is generally unreliable and results obtained using this method cannot be used interchangeably with mechanomyography [3]. It is believed that there is currently no electromyographic monitor used in clinical routine.
Acceleromyography measures the acceleration of movement created by muscle contraction. Acceleromyography has been used in research and clinical routine for more than a decade and there are still fundamental problems which have inhibited widespread use of this technique. For example:                This technique can only be used to measure neuromuscular blockade at the adductor pollicis muscle and not reliably at any other muscle (e.g. eye muscles) [4];        The results obtained using this technique are dependent on the exact position of the hand; and        the use of this technique is cumbersome and commercially available monitors give very little information, especially no display of the original evoked signals.        
Especially the eye muscles are prone to these problems: the corrugator supercilii muscle is an interesting small muscle that is responsible for vertical frowning. The corrguator supercilii muscle correlates well with the adducting laryngeal muscles in terms of onset and offset of neuromuscular blockade [5]. For clinical routine, acceleromyography has been well established as a method to monitor neuromuscular blockade at the adductor pollicis muscle [6]; however, there are some studies questioning its validity in comparison to mechanomyography [7] and it is mostly used to monitor recovery from neuromuscular blockade [8].
Because of the above technical problems, most clinicians still rely on the simple tactile or visual estimation of neuromuscular blockade. Although this might be clinically acceptable when recovery from operation block is judged, it is clearly not objective and cannot be used to titrate neuromuscular blockade during surgery.
The present invention provides a method and device using phonomyography for conducting neuromuscular monitoring, in order to overcome at least in part the drawbacks and limitations of the above discussed prior devices and methods.